In the past, diabetics have used individual test strips for self-diagnosis in which the strips were analyzed photometrically after applying a small amount of blood to the strip to determine the glucose content of the fluid sample as accurately and reliably as possible. In order to improve the user-friendliness of this test system, it has been proposed that such testing be performed on a test tape in the form of a tape cassette. It is possible to insert tape cassettes as a disposable component into compact hand-held devices to automatically and rapidly carry out all required analytical steps. In these tape cassette testing systems, the disposable components are mass produced and high demand occurs due to the reliability.
Accordingly, document WO 2005/006985 discloses a test tape guide curved in an arch that exposes a section of test tape to receive an application of liquid on the front side and to perform an optical measurement on the rear side by means of a reflection-photometric measuring unit focusing on this area. However, with this arrangement having parallel arched rails, there is a risk that in the case of a thin flexible test tape the central tape area will arch inwards under tension in the free space between the rails and thereby making an accurate optical focus more difficult. Such tape deformation has already proven to be problematic in the case of small radii of curvature, especially with regard to measuring optics having a short focal distance.
In order to overcome this problem, it has been proposed that optical elements in the form of cylinder lens be used as an application tip, wherein the lens bundles the measuring light in the optical path of the photometer. However, just as in the case of simple optical windows, care must be taken that high quality requirements are achieved for the required measuring accuracy especially with regard to transmission, scratch resistance, temperature resistance, coefficient of expansion, optical quality, and other material parameters or faults such as those listed in International Standard ISO 10110.